Monogamy for lesbians, committed but moderately open relationships for straights, and no strings free-for-all for gay men. The amount of faithful monogamy sustainable in a relationship is directly proportional to how female the relationship demographics are. A relationship that's only 50% female is unlikely yo stay utterly monogamous. A relationship that's 0% female? I wouldn't bet a Saltine cracker on that staying monogamous, and I don't even like Saltines.
